Senile dementia of the Alzheimer type is probably the most common disabling neurologic disorder of older adults. It is the major cause of dementia identified in hospital and necropsy studies. Considered to be 4th or 5th most common cause of death in older Americans, it is the most frequent reason for their referral to nursing homes. Though it will affect an estimated 5 million Americans by the year 200, if English estimates apply, its prevalence in noninstitutionalized individuals in the U.S. is presently unknown. Cause is unknown. The clinical disorder affects only man. Early diagnosis is essential to participation, collection of medical and family history, and careful neurologic and neuropsychologic evaluation. Identification of early cases will minimize bias inherent in premature death or withdrawal from the community of severely affected individuals and confusion between disease precursors and sequelae. Early identification is also essential to trials of pharmacologic or psychologic intervention with patient or family. Among practical problems in human study have been the lack of an early testable definition, sensitive, simple and operational diagnostic techniques and access to large numbers of potentially affected non-institutionalized persons. We have developed a working definition, concurrently validated a brief economic cognitive function test and a serially branching survey diagnostic tool. We have almost completed the first step of instrumental validation, a study of mildly affected members of the population referred by local physicians. This study suggests an almost 10-fold increase in predictive value positive over brief, older mental function tests when the cognitive function instrument is used as a screening device. Compared with the diagnoses of two experienced clinical neurologists, the instrument will have sensitivity of approximately 93% and specificity of approximately 95%. Evaluation of a representative sample of a large defined population will complete the validation and provide preliminary estimates of prevalence in that population. Population-specific neuropsychological data are also being obtained.